The invention relates to a method for reheating cleaned gases subsequent to wet-process cleaning of raw gases by means of a regenerative heat exchange between the raw gases and the cleaned gases, in which a previously reheated partial flow of cleaned gas is added to the flow of cleaned gas before the regenerative heat exchange is performed.
In the desulfurization of combustion gases of combustion systems in a wet-washing cleaner, it is known to reheat the washed clean gases to higher temperatures in a rotating regenerative heat exchanger counter to the incoming hot raw gases, in order to bring them to the temperature required for their rise in the exhaust flue. In the gypsum method, the disadvantage arises that droplets of the washing medium are carried away with the moist, saturated cleaned gases emerging from the wet washing process, causing gypsum contained in the washing medium to be carried into the regenerative heat exchanger; the gypsum precipitates onto the heat exchanging surfaces of the heat exchanger in the form of a coating that can be dissolved only with difficulty. It has accordingly already been proposed that the washed clean gases be predried, before they enter the gas heat exchanger, by means of adding a partial flow of raw gases without supplying external energy. Thus the water content of the droplets evaporates, and the gypsum dust, now dried, can easily be removed from the heat exchanging surfaces using known agents. However, because of the admixture of raw gas, sulfur is reintroduced into the cleaned gases, which lowers the degree of desulfurization attained. This disadvantage is avoided by means of a modification (German Patent Disclosure Document DE-OS No. 32 25 716) of this known method such that, instead of the partial flow of raw gas, a partial flow of the cleaned gases already reheated in the heat exchanger is diverted and recirculated, by means of a pressure-increasing blower, into the cleaned gases supplied to the heat exchanger by the wet washer. The quantity of recirculated cleaned gas is regulated here in accordance with the difference in temperature between the cleaned gases at the outlet of the gas washer and the gases prior to the entry into the gas heat exchanger, after the recirculated partial flow has been added.
Another solution is also known (German Patent Disclosure Document DE-OS No. 29 00 275), in which a partial flow is diverted from the total flow of cleaned gases, heated in a rotating regenerative heat exchanger by means of the total flow of raw gases before their entry into the wet washer, and reintroduced into the main flow of cleaned gases before the partial flow of the cleaned gases is removed. Because of the preceding indirect heat exchange between the recirculated partial flow of cleaned gas and the total flow of raw gases, the raw gases are delivered to the wet cleaner having been cooled by the transfer of heat to the partial flow of cleaned gas. The highly heated partial flow of cleaned gas then exchanges heat, in a gas mixer, with the total flow of cleaned gases directly leaving the washer and heats this total flow to such an extent that the cleaned gases enter the exhaust flue at the temperature required to make them rise in it.
Although in the last two known methods described above no raw gases are admixed with the cleaned gases, a certain--albeit limited--quantity of raw gas is introduced into the cleaned gas side of the heat exchanger as a consequence of the use of rotating regenerative heat exchangers, so that small quantities of sulfur are still contained in the cleaned gases emerging from the gas heat exchanger.